It is well known in the art that along the path of a power grid there are provided several electric substations for transmitting and distributing electricity from a power generating source to loads and users connected to the feeding grid; these substations may be configured according to different layouts depending on the applications, for example in/out or T-type layouts, H-type layouts et cetera, and are realized by using a series of electric components, such as disconnectors, circuit breakers, instrument transformers, control systems.
According to more traditional solutions, electric substations have been realized by using several components which are structurally independent and suitably connected to each other and to the power line when assembling in order to obtain the required layout and to perform each a respective dedicated function; these traditional solutions have presented some drawbacks in practical use, mainly due to the large number of components required, even for providing a minimal configuration, and to their structural and functional separation. Indeed, these aspects result in heavy maintenance requirements for each and any of the single components used, and to a considerable increase of the overall dimensions of the substation, with a consequent negative impact on installation and maintenance costs, as well as on environmental impact.
To overcome the above mentioned drawbacks, in recent years some new compact gas-insulated switchgear devices have been designed, which integrate in a unique apparatus and are able to perform several electrical functions which, in the more traditional substations, were obtained by using multiple structurally separate elements.
In particular, such devices comprise an external casing having a pod-shaped portion on which there are mounted two or three bushings each containing a corresponding electric terminal for input/output connections with a power line and/or other elements of the substation; inside the pod-shaped portion there are normally provided at least a disconnection unit and an interruption unit, which are suitably conceived and electrically connected to each other and to the electric terminals, in such a way to perform electrical disconnection or circuit breaker maneuvers, respectively.
Examples of such kinds of gas-insulated switchgear devices are disclosed in the international patent application Ser. No. WO0024099 and U.S. Pat. No. 5,796,060.
This type of switchgear devices has significantly contributed to reduce the number of components needed, thus allowing to realize more compact substations with less environmental visual impact, and installation and maintenance costs reduced; nonetheless, these devices still present some aspects which may be further improved as regard to their structure, flexibility in the number of electrical maneuvers which can be executed, especially with respect to the number of possible combinations and coordination of circuit breaker and disconnector operations, and the way to perform them too. For example, before operating the disconnector unit for disconnecting one or more of the input or output connections, it is usually necessary first to open the interruption unit thus breaking the current flow and therefore putting out of commission the whole device; this is not entirely satisfactory for example when the device has an input connection and two output connections only one of which should be disconnected e.g. for maintenance reasons, while the other connections with the loads connected could be kept working.